The present invention relates generally to grain drying systems and more particularly to an automated method and control system for achieving a target grain moisture content.
The moisture content of freshly harvested grain must be lowered before storage to prevent spoilage. A moisture content of 15 percent is considered optimum by commercial grain elevator operators for long term storage of the grain when the grain is exposed to air, and 14 percent is preferred to retard spoilage of stored grain where access to air is limited. The removal of too much moisture is wasteful of drying energy and usually results in shrinkage which decreases the value of grain. If the moisture is removed too rapidly, damage to the grain results. A continuous flow grain dryer is typically operated by forcing high amounts of heated air through a moving layer of grain in a grain dryer. Typical grain dryers and control systems thereof are disclosed in U.S. Pat. Nos. 4,194,844, 4,750,273, 5,570,521, and 5,651,193, the disclosures of which are hereby incorporated by reference in their entirety.
A grain dryer can be configured to heat grain to drive off unwanted moisture then to cool the grain. Alternatively, full heat drying can be employed whereby grain is heated within a grain dryer to a preselected temperature or moisture content and discharged to cool outside the dryer. Some moisture will be driven out of the kernels as the grain cools. This amount of moisture reduction is predictable as long as the cooling time is consistent and several hours in length. The amount of moisture reduction in grain per unit of temperature reduction as it cools is an essentially constant value for most grains and is known as a bin factor. The specific bin factor for a drying operation is dependent on the ambient temperature and humidity (i.e. one would expect that a bin factor for a moist climate, such as Quebec, would differ from a bin factor from a similar drying operation in a dryer climate, such as Nebraska).
Typical automated control systems for full heat dryer operations use either grain moisture or temperature to determine when grain is removed from the dryer. In a moisture based system, an automated controller detects the grain moisture content and the grain is discharged when a target moisture content is met. In a temperature based system, the temperature of grain near the discharge is determined and the discharge rate is adjusted if a pre-selected temperature is not met. A typical moisture based control system is disclosed in U.S. Pat. No. 4,916,830.
For both temperature and moisture based systems, an operator manually factors in the bin factor, i.e., the additional amount of moisture that should be removed from the grain in the bin per unit temperature during the cooling process. In both systems, the operator must be capable of measuring or approximating data about the moisture content and temperature at the discharge to calculate the final moisture content expected in the bin after the grain cools down. This information is used to manually adjust the target discharge temperature or moisture contentxe2x80x94as the case may bexe2x80x94to account for the bin factor. Typically, this adjustment to a grain dryer automatic controller is made when grain from a different field is fed into the dryer, or when a grain with a variable moisture content is expected.
The bin factor that is used in conventional grain drying operations is selected from charts or from historical data from a previous, similar operation. Typically, an operator will measure the moisture content and temperature of a sample of grain both as it is discharged from a grain dryer and after cooling. By dividing the change in moisture content by the change in temperature, an approximate bin factor can be determined and used for subsequent drying operations. For corn, a bin factor of 0.5 percent of moisture reduction per 10 degrees Fahrenheit (above 80xc2x0 F.) is typical. As an example, in a moisture based system, the operator desires a moisture content of 14%. Using a historical bin factor of 0.5 percent moisture reduction per 10 degrees Fahrenheit, an estimated grain dryer discharge temperature of 140xc2x0 F., and an ambient, or control, temperature of 80xc2x0 F., the operator manually calculates an expected moisture reduction of 3% [(0.5 percent moisture reduction per 10 degrees Fahrenheit) multiplied by ((140xc2x0 F.-80xc2x0 F.) divided by 10)]. The operator then adds this expected 3% to the target 14% to arrive at a 17% moisture content desired at the dryer discharge. The operator then adjusts the dryer controller to discharge grain at 17% moisture content. This method would be adequate if the grain were to discharge from the dryer at a consistent 17% moisture content and 140xc2x0 F. In operation, a conventional moisture based system will discharge grain at a moisture content of 17%, regardless of temperature.
With moisture based control, grain discharged with a consistent moisture content may vary considerably in temperature. With a constant bin factor, kernels will emit different amounts of moisture while cooling, resulting in cooled grain that varies considerably in moisture content. In temperature based control, grain may leave the dryer with a consistent temperature, but vary considerably in moisture content. With a constant bin factor, all kernels will emit identical amounts of moisture, resulting in grain that also varies in moisture content.
These problems are particularly acute with newer, hybrid grains which tend to exhibit considerable variation in as-harvested moisture content and drying rate. In addition to the ambient conditions that affect cooling moisture reduction rates mentioned previously, inconsistent drying rates both within the dryer and during cooling are also associated with variances in maturity, seed coat permeability and thickness of the grain. Thus, unless an operator is prepared to continually monitor the output data and make the bin factor adjustment throughout the drying process, variations in the final moisture content in the storage bin will occur.
What is needed is a completely automated method whereby grain can be discharged from a grain dryer to achieve a preselected moisture content or moisture content range in the grain storage bin, without the need for manual calculation with respect to the bin factor.
The present invention is directed to a moisture based grain dryer control system and method that uses the dryer outlet grain temperature to account for additional moisture reduction during cooling. In one aspect of the present invention, a method is provided wherein grain moisture content and temperature within a grain dryer is periodically detected and the expected moisture removal for the corresponding temperature and bin factor are calculated. Grain is discharged from the grain dryer as the expected post cooling moisture content added to a desired target moisture content is within a tolerance of the measured moisture content within the dryer.
In another aspect, the present invention provides an electronic system whereby a dryer module receives input from a user interface and various temperature and moisture content sensors of a grain dryer. The dryer module periodically calculates an expected post cooling moisture content and generates a signal to control the rate of discharge of grain to a measured moisture content and temperature sufficient to dry down while cooling to the target value.
In another aspect, the present invention provides a control system for a grain dryer wherein parameters of a material being dried are measured and a known target moisture content is compared to a measured moisture content and an expected cooling bin moisture reduction amount.
In yet another aspect, the present invention provides a grain dryer wherein a control system measures parameters of a material being dried and a known target moisture content is compared to a measured moisture content and an expected cooling bin moisture reduction amount.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.